Rotating or permanent night work cause dissociation of rhythms from the 24h light/dark phases with resultant poor sleep, diminished alertness and mood disturbances. The project tests the hypothesis that melatonin acclerates the adjustment of workers to a reversal in the activity/sleep cycle from daytime to night work by synchronizing sleep to the desired schedule and consequently improving alertness and mood during the waking hours. The hypothesis will be tested in an actual work place, a hospital. Subjects will be physicians in training whose work requires intense alertness and vigilance. The design includes two treatment phases for each subject, melatonin and placebo, and two respective baseline phases, one for each treatment; each phase lasts two weeks.